Strengthening and fracture mechanisms of a precipitation hardening high-entropy alloy fabricated by selective laser melting

A precipitation hardening high-entropy alloy (HEA) (FeCoNi)86Al7Ti7 was fabricated by selective laser melting (SLM) and ageing treated under different temperatures and time conditions. Yield strength of the aged HEA increases substantially from 710 to 934 and then to 1203 MPa. Theoretical analyses reveal that the coherent L12 precipitate contributes most of the improved strength for the aged HEAs, whereas recovery during ageing causes the decrease of dislocation density thus exerts a softening effect. In addition, it is found that ductility decreases with increasing volume fraction of the incoherent L21 precipitates. Based on a void growth model, the trend is qualitatively explained. Moreover, a new fracture mode, intercellular fracture, is proposed to account for the strong dependence of fracture dimple size on the dislocation cells, also directly validated by delicate microstructural examination. The findings provide an effective strengthening method and propose a unique fracture mechanism for the additively manufactured HEA.